This invention relates to an apparatus and method for anchoring a bolt in a rock formation, and, more particularly, to a reinforcing bar assembly adapted for use with a bonding material.
It is well-known in the art of mine roof support to utilize systems which combine a mechanical or conventional bolt and resin anchoring. A typical reinforcing bar (rebar) system includes a rebar with external threads on one end, and externally threaded bolt carrying plate for engagement with the roof surface, and an internally threaded coupling device for joining the bolt and rebar. The diameters of both the rebar and bolt threaded portions usually differ such that their advancement in the coupling is limited. While the rebar is completely engaged with the coupling, the bolt normally advances only a fraction of the way in the coupling, its axial movement arrested by a stop means in the form of a shearable pin or deformed thread.
Once assembled, this system is inserted into a bore hold behind a resin capsule so as to rupture the capsule causing the enclosed resin and catalyst to engulf the rebar and the coupling. Mixing of the resin components is accomplished by rotating the bolt, thereby causing the rebar/coupling assembly similarly to rotate. The mixture is then permitted to cure and harden. After the resin cures and hardens, rotation of the rebar and coupling is resisted, thereby allowing the torque applied by the bolt to the stop means to exceed a predetermined level. By this action, the bolt displaces the shearable pin (or overcomes the resistance of the deformed thread), thereby continuing its axial movement within the coupling. Upon continued rotation, the bolt can be tensioned in the bore hole.
One of the major shortfalls of these systems lies in the construction of the stop means. The most familiar stop means is in the form of a shearable pin which breaks upon application of the preselected torque. Severe problems exist in practice, however, in determining the proper material and associated pin dimensions for constructing a pin which reacts to stress as planned. As a result, there have been problems in the operation of these systems, with many systems failing to operate properly in actual use.
Therefore, the principal object of the present invention is to provide a method and apparatus for combining resin bonding and mechanical tensioning of a bolt in a rock formation by a rebar assembly provided with a resistance means which will thread when a torque in excess of a predetermined value is continuously applied to a bolt abutting the resistance means.
Another object is to provide a resistance means which is tapped by a bolt upon continuous application to the bolt of a torque in excess of a predetermined torque.
Still another object is to provide an apparatus which employs only two components for ease of installation and use.